http://en.wikipedia.org/wiki/X
In
English orthography, x typically represents the voiceless consonant cluster
/ks/ when it follows the stressed vowel, and the voiced consonant
/ɡz/ when it precedes the stressed vowel (e.g. 'exam'), or when it precedes a silent 'h' and an accented vowel ('exhaust').
[2] As the first letter of a word, it usually represents
/z/ (e.g. 'xylophone', 'xenon'), and in compounds keeps the
/z/ sound, as in (e.g. 'meta-xylene'). Before 'i' or 'u' it can also represent the sounds
/kʃ/ or
/ɡʒ/ for example, in the words 'sexual' and 'luxury', respectively: these result from earlier
/ksj/ and
/ɡzj/. It also makes the sound
/kʃ/ in words ending in -xion (typically used only in
British-based spellings of the language; American spellings tend to use -ction). Word-final 'x' is always
/ks/ (e.g. 'ax'/'axe') except in loan words such as 'faux' (see French, below).
In abbreviations, it can represent "trans-" (e.g. XMIT for transmit, XFER for transfer), "cross-" (e.g. X-ing for crossing; XREF for
cross-reference), "Christ" as shorthand for the
labarum (e.g. Xmas for Christmas; Xian for Christian), the "Crys" in Crystal (XTAL), or various words starting with "ex" (e.g. XL for extra large; XOR for
exclusive-or).